In cryogenic processes it is necessary to remove carbon dioxide, water and other high boiling materials from the feed gas stream since they will liquefy or solidify at low temperatures leading to pressure drops and flow difficulties in the downstream process. It is also desirable to remove hazardous and/or explosive contaminants from the feed gas to reduce the risk of build-up in the subsequent processes. These contaminants are typically removed using cyclic adsorptive gas purification processes.
Cyclic adsorptive gas purification processes typically employ one of two general classes of adsorption systems, namely: temperature swing adsorption (TSA) systems and pressure swing adsorption (PSA) systems. These adsorption systems typically contain two or more adsorbers that contain adsorbents for removal of impurities from a feed gas. The adsorbers are usually described to be operating in a production state also referred to as an adsorption state or in the regeneration state. The adsorber in the production state is also referred to as being online. The adsorber in the regeneration state is also referred to as being offline. In the production state of both TSA and PSA systems, a feed gas stream is contacted with an adsorbent bed in the adsorber to produce a purified gas stream. The adsorber may contain one or more adsorbents. A given adsorbent selectively adsorbs one or more impurities present in the feed gas stream. In processes where air is the feed gas, water and carbon dioxide are typically removed by contacting the feed gas with one or more adsorbents which adsorb water and carbon dioxide. The water adsorbent material typically is silica gel, alumina or a molecular sieve and the carbon dioxide adsorbent material typically is a molecular sieve, for example, a zeolite. Water is typically removed first, followed by carbon dioxide by passing the feed stream/air through a one or more adsorbent layers chosen for their selectivity in adsorbing water and carbon dioxide. At the end of the production state, the flow of feed gas to the adsorber is shut off. In the regeneration state of both TSA and PSA systems, the contaminant laden adsorbent bed is exposed to a flow of regeneration gas which facilitates desorption of impurities from the adsorbent and removal of desorbed impurities out of the adsorber (e.g. carbon dioxide and water to regenerate the adsorbent material for further use. The regeneration gas in the regeneration state conventionally flows in a direction counter current to that of the feed gas flow in the production state. In the TSA system, the regeneration gas employed is a heated regeneration gas, provided at a temperature higher than that of the feed gas. Typically the temperature of heated regeneration gas is in the range of about 200° F. to about 600° F. The heated regeneration gas heats the adsorbent and facilitates regeneration of the adsorbent by desorption of impurities. The adsorbent has a lower adsorptive capacity at higher temperature. The heated regeneration gas serves as a hot purge gas that removes the desorbed impurities from the adsorber. This is then followed by a cooling step that involves flowing a near ambient temperature regeneration gas to cool the adsorbent, push out the heat front through the adsorbent bed, and make it ready for adsorption step. The PSA system in an air separation plant employs a waste gas stream produced within the air separation plant as regeneration gas. The waste gas is typically at a temperature close to the feed air temperature, and is provided to the PSA system at a pressure above the atmospheric pressure, sufficient to overcome the pressure drops and to be able to be discharged to the atmosphere. The adsorbed impurities in the PSA are desorbed due to the lower adsorptive capacity at lower pressures. The PSA regeneration gas serves as a purge gas that facilitates the regeneration of the adsorbent by desorption of impurities and removal of the desorbed impurities from the adsorber.
In cryogenic air separation plants, the cyclic adsorptive gas purification system can contain one or more adsorbers, referred to as prepurification units or prepurifiers, and produce purified air for distillation at cryogenic temperatures by adsorbing impurities in feed air. By using at least two adsorbers in a parallel arrangement, the cyclic adsorptive gas purification system can be operated in a continuous mode; for example one adsorber can be operated in an adsorption state while the other adsorber is being regenerated and their roles are periodically reversed in the operating cycle, with equal periods being devoted to the adsorption state and to the regeneration state. Typically, such systems contain adsorbers that are substantially cylindrical in shape, and may have their axis with respect to feed flow as axial (vertical or horizontal), or of the radial type.
A conventional TSA process cycle for purifying air is generally described to contain the following steps: a) production of purified air by adsorption of impurities in feed air flowing through an adsorber at super atmospheric pressure and at ambient temperature for a pre-determined time period;                b) initiating regeneration of the adsorbent by stopping the feed air flow and depressurizing the adsorber to a lower operating pressure, typically near atmospheric pressure;        c) regeneration of the adsorbent in the depressurized adsorber by flowing a heated regeneration gas also referred to as hot purge gas for a pre-determined time period; an example of a heated regeneration gas is waste nitrogen produced in the air separation unit that is heated by means of one or more heaters/heat exchangers;        d) cooling the regenerated adsorbent in the adsorber to push out residual heat in the adsorbent bed by flowing cool waste gas;        e) repressurizing the adsorber with purified air coming, for example, from another adsorber in the production phase;        f) bringing the repressurized adsorber onlineand repeating steps (a) thru (e). Less conventionally, the regeneration may be carried out at a pressure substantially different from atmospheric pressure, either greater or even less than the ambient pressure by using suitable vacuum pumping means.        
A conventional PSA process cycle for purifying air is usually described to contain:                a) production of purified air by adsorption of impurities in feed air flowing through an adsorber at super atmospheric pressure for a pre-determined time period;        b) initiating regeneration of the adsorbent by stopping feed air flow and depressurizing the adsorber to a lower operating pressure, typically near atmospheric pressure;        c) regeneration of the adsorbent in the depressurized adsorber by flowing a purge gas for a pre-determined time period; an example of a purge gas is waste nitrogen produced in the air separation unit;        d) repressurizing the adsorber with purified air coming, for example, from another adsorber in production phase;        e) bringing the repressurized adsorber online,and repeating steps (a) thru (d). The PSA process cycle is distinguished from the TSA process cycle in that the regeneration gas is not heated. Adsorbent bed cooling step is not required since the adsorbent doesn't get heated by the regeneration gas. The PSA cycle time is typically much shorter compared to the TSA cycle time.        
Extreme instantaneous variations in ambient contaminant levels, for example CO2, can occur in highly populated regions and areas of dense industrial activity. Variations in contaminant levels can lead to significant and sometime unexpected contaminant breakthrough at the outlet of the prepurifier. Contaminant breakthrough can occur even in cases where the average contaminant level observed during the feed step is lower than the average contaminant level the prepurifier was designed to handle and where the feed step duration is shorter than design. Depending on the magnitude of these ambient contaminant spike events, a plant trip can occur. If too frequent, those contaminant breakthrough events can lead to plant shutdown.
This invention proposes to integrate a control of a thermal, pressure, and/or hybrid swing adsorption prepurifier operation cycle time by monitoring the ambient contaminant levels being fed to a prepurifier in order to avoid contaminant breakthrough at the outlet of the prepurifier caused by extreme instantaneous variations in ambient contaminant level.